Injection of anhydrous ammonia into the soil is a commonly used method of supplying nitrogen fertilizer to grain and other crops using an applicator vehicle pulled by a tractor. An ammonia storage tank is pulled behind the applicator. A hose connects the storage tank to the distribution system on the applicator. The distribution system splits the ammonia into separate lines which feed several knives. These knives are lowered into the soil several inches and ammonia is injected into the ground at the bottom of the knives as the knives are pulled through the soil.
Anhydrous ammonia is also used as a commercial refrigerant. Pressurized liquid ammonia assumes a temperature of about −28° F. when it is depressurized to atmospheric pressure. As it loses pressure, part of the ammonia liquid evaporates, the heat of evaporation cooling the resulting vapor and liquid. The ratio of vapor to liquid is determined solely by the saturation temperature at the original pressurization and the ammonia's current pressure. The fact that anhydrous ammonia is an excellent refrigerant causes a problem when it is being applied as fertilizer in a field. By the time the ammonia reaches the knife or other injection device it has depressurized to near atmospheric pressure and is very cold. The injection tube is attached to the knife which opens the soil. Heat transfer between the tube and the body of the knife is such that, under some conditions, soil and crop residue will freeze to the knife.
Typically, the anhydrous ammonia is divided among the multitude of knives or other injection devices at one or more manifolds. Plastic tubing is used to connect the manifold outlets to the steel tubes attached to the knives. These tubes are typically ⅜ to ½ inch outside diameter with a wall thickness of over 1/16 inch. Most commonly, the plastic tubing is made from EVA copolymer, non-reinforced. The tubing is held in place by hose clamps.
There are at least two systems in commercial use which transport anhydrous ammonia by small diameter, high pressure plastic tubing from manifolds to the injection devices at the soil. One is manufactured by Exactrix Global Systems of Spokane, Wash. The other is manufactured by a NH3.Company of Quincy, Ill. Typically, the high pressure plastic tubes extend through metal injection tubes, terminate at the exits of the injection tubes and are restrained so that they do not move with respect to the injection tubes. These systems require the use of long sections of the small plastic tube to connect from the manifolds through the injection tubes touching the soil. Discharge orifices are provided on the discharge end of each small plastic tube to keep anhydrous ammonia at an elevated pressure and warm. An advantage of this approach is that the small plastic tubing has a small vapor gap between it and the larger steel injection tube which goes to the soil. Additionally, the plastic tube has poor thermal conductivity. The combination of air gap and poor thermal conductivity reduces heat flow from the body of the knife to the anhydrous ammonia stream in the injection tube. This prevents or minimizes the freezing of soil and crop residue to the knife. High pressure systems may leak expensive ammonia and can result in injury to individuals using the high pressure systems.
More commonly, anhydrous ammonia systems use ⅜″ I.D. or ½″ I.D. clear EVA copolymer non reinforced plastic tubing to connect to the injection tubes, the inside diameter of the tubing being determined by the outside diameter of the injection tube. The wall thickness of the EVA copolymer tubing is generally 1/16 inch or greater. Pressures carried by this tubing are low, generally less than 60 psig. This method causes the steel injection tubes to become very cold since the ammonia passing through them is near atmospheric pressure and approaching the temperature of ammonia at atmospheric pressure, about −28° F. Under some conditions when these very cold tubes transmit heat from the bodies of the knives, soil and crop residue freeze to the knives.